1. Field of the Invention
The present invention relates to a system for distributing picture data and in particular relates to a system for distributing a plurality of picture data from a picture distribution device to a plurality of picture receiving devices by a time division multiplex method using a ring-shaped transmission line.
2. Description of the Related Art
A system for displaying pictures taken by a plurality of cameras at a plurality of terminals is widely used in a variety of fields. For example, a system in which monitor cameras are installed at a plurality of locations, the picture from each monitor camera is transmitted to a central station via a network, the situation of each location is monitored by displaying pictures transmitted from the plurality of locations on a plurality of monitors installed in the central station is widely known. As specific examples, a road traffic monitoring system, disaster situation monitoring system, etc., are widely known. In the road traffic monitoring system, a traffic condition is monitored using pictures transmitted from each monitor point, and traffic guidance information is provided based on the pictures. In the disaster situation monitoring system, monitoring cameras are installed in a river which is in danger of flooding, etc., and an alarm is issued based on pictures transmitted from the cameras.
FIG. 1 shows the configuration of an example of an existing picture distribution system. This system comprises a plurality of cameras 101a-101c, a distribution device 102 for transmitting picture data outputted from each of the cameras 101a-101c to a network, a plurality of receiving devices 103a-103c for receiving the picture data from the network and a plurality of picture monitors 104a-104c for displaying the picture data received by corresponding receiving device. In this case, the network is a ring-shaped transmission line 105. A plurality of logical channels #a-#c are established in the transmission line 105.
Each of the receiving devices 103a-103c receives picture data from a respective predetermined logical channel. In the example shown in FIG. 1, the receiving devices 103a-103c receive picture data from the logical channels #a-#c, respectively.
The distribution device 102 transmits picture data outputted from each of the cameras 101a-101c to a corresponding logical channel. In the example shown in FIG. 1, a distribution request “to display picture data taken by a camera 101a on a picture monitor 104b and to display picture data taken by a camera 101b on picture monitors 104a and 104c” is issued to the distribution device 102. Therefore, the distribution device 102 transmits the picture data taken by the camera 101a to a logical channel #b and transmits the picture data taken by camera 101b to logical channels #a and #c. A distribution request is, for example, issued from the central station, which is not shown in FIG. 1.
According to the system described above, pictures taken by a plurality of specific cameras can be displayed on a plurality of respective supervisory monitors.
In the system described above, the transmission method of picture data is not limited to the method described above. However, in a system for monitoring the traffic condition of a road, the situation of a river, etc., it is anticipated that a picture taken by each camera is displayed for a fairly long time. In this case, the amount of picture data transmitted from each camera to a supervisory monitor does not vary greatly as time elapses. Therefore, such a system often adopts time division multiplexing as a transmission method of picture data.
In a time division multiplex method, picture data are usually stored in a fixed-length frame composed of a plurality of time slots and transmitted. In this case, as shown in FIG. 2, each logical channel usually corresponds to one or a plurality of time slots. In an example shown in FIG. 2, time slots #1-#3 for logical channels #a-#c are provided for each frame. In this case, for example, data to be stored in the time slot #1 are transmitted via the logical channel #a. The length of each time slot is fixed in advance.
If a frame as shown in FIG. 2 is used, the distribution device 102 stores picture data taken by the camera 101a in the time slot #2 and stores picture data taken by the camera 101b in the time slots #1 and #3. Each of the receiving devices 103a-103c extracts picture data from the time slots #1-#3, respectively. In this way, pictures taken by camera 101a are displayed on the picture monitor 104b, and pictures taken by camera 101b are displayed on the picture monitors 104a and 104c. 
In the system described above, each receiving device is connected to a predetermined logical channel. In the example shown in FIG. 1, the logical channels #a-#c are fixedly connected to the receiving devices 103a-103c, respectively. Specifically, the receiving devices 103a-103c can receive only picture data transmitted via the logical channels #a-#c, respectively.
Therefore, in order to display the same picture on a plurality of picture monitors, the same picture data must be transmitted via the number of logical channels equal to the number of the picture monitors. In this case, a plurality of logical channels are used by a plurality of picture data that is the same. In the example shown in FIG. 1, the two logical channels #a and #c are occupied by picture data outputted from the camera 101b. As a result, the efficiency of use of communications resources (the band of a transmission line 105) is degraded.
If the display of a picture monitor is switched, sometimes the display of another picture monitor may also be simultaneously switched.
Furthermore, in a configuration such that logical channels are connected to receiving devices on a one to one basis, the number of picture monitors which can be connected to this system is restricted by the number of logical channels established on the transmission line 105.
As described above, if the existing picture distribution system adopts time division multiplexing, the efficiency of use of communications resources is low and the number of picture monitors used to display pictures is restricted.